Carboxylic acids are commercially important. They can be used as ingredients for making plasticizers, lubricants, dielectric fluids, and synthetic fibers. The short-chain aliphatic carboxylic acids are colorless liquids, each of which has a characteristic odor that is often sharp or penetrating. For example, the distinctive taste and odor of acetic acid make it an important flavoring agent.
Fatty acids are long chain (generally about C.sub.4 or greater) carboxylic acids. Naturally occurring fatty acids generally have 10 to 22 carbons. Some fatty acids have commercial uses. For example, oleic acid can be used for making plasticizers, coatings, etc. The higher fatty acids are generally derived from animal oils, vegetable oils, and fish oils. Often, when an oil is hydrolyzed, a mixture of fatty acids is formed. Such mixtures usually contain both monoethylenically unsaturated and polyethylenically unsaturated fatty acids. Generally unsaturated carboxylic acids can be oxidatively cleaved to form smaller molecular weight carboxylic acids.
A number of methods have been disclosed to oxidatively cleave the double bonds of unsaturated compounds to form carboxylic acids. In these reactions, oxidants and catalysts are generally required. Olin et al. (U.S. Pat. No. 2,226,357) disclose a method for producing polycarboxylic acids by treating cyclo-aliphatic olefins with sulfuric acid and subsequently treating the sulfation product with nitric acid or other oxidants in the presence of a catalyst such as vanadic acid.
Ellingboe (GB Patent No. 524,163) and U.S. Pat. No. 2,203,680), discloses a method for producing saturated aliphatic carboxylic acids by the oxidation of long chain unsaturated fatty acids, such as oleic acid, erucic acid, linoleic acid, and linolenic acid, with highly concentrated nitric acid. The oxidation reaction is catalyzed by a catalyst, namely a vanadium compound such as ammonium vanadate.
Although the above methods can be used for producing carboxylic acids, the use of nitric acids is problematic. Often, toxic gases (NO.sub.x), as well as other undesirable oxidation side-products are produced. The NO.sub.x gases present a health problem. Further, processing necessary to remove the undesirable side products results in increased cost of production for the desired products.
Other oxidants have also been used for the oxidative production of carboxylic acids. Ralston et al. (U.S. Pat. No. 2,133,008) disclose a process for cleaving unsaturated fatty acids, glycerides thereof, and water soluble soaps thereof by an alkali metal hypochlorite in the presence of a hypochlorite-decomposing agent. The suitable hypochlorite-decomposing agents disclosed include iron, cobalt, manganese, and nickel compounds. Dunlop et al. (U.S. Pat. No. 2,676,186) disclose the oxidation of levulinic acid to succinic acid. In this reaction, vanadium pentoxide is used as a catalyst, and air or other oxygen-containing gas is used as an oxidant. Zaldman et al., J. AM. Oil Chem. Soc., 65:611-615 (1988) disclose the use of oxidants, such as ozone, sodium hypochlorite, potassium permanganate, potassium bichromate, chromic acid, and hydrogen peroxide, for the oxidation of unsaturated fatty acids.
Several methods based on the use of hydrogen peroxide as the oxidant for producing carboxylic acids are known. For example, Nakazawa (U.S. Pat. No. 4,833,272) discloses the use of hydrogen peroxide in oxidizing a Dieis-Alder reaction product of maleic anhydride and a diene and/or the corresponding acid catalyzed by at least one of tungstic acid, molybdic acid, heteropoly acids thereof, and mixed coordination heteropoly acids. Nakazawa et al. (U.S. Pat. No. 4,606,863) disclose a process for preparing carboxylic acid by subjecting an oxidation product produced by reacting a peroxide with an unsaturated aliphatic monocarboxylic acid or aliphatic olefin to oxidation by oxygen or oxygen-containing gas in the presence of a catalyst. Other than hydrogen peroxide, a few other hydroperoxides are also disclosed.
A variety of catalysts has been used to catalyze oxidative reaction to produce carboxylic acids. Ellingboe (GB Patent No. 524,163 and U.S. Pat. No. 2,203,680) disclose the use of ammonium vanadate catalyst with nitric oxide as the oxidant. Olin et al. (U.S. Pat. No. 2,226,357) disclose the use of a catalyst such as vanadic acid, manganese, chromium, vanadium, tungsten, or compounds thereof, such as nitrates, oxides, or sulfates, in a nitric acid solution.
Drent (U.S. Pat. No. 4,868,328) discloses a process for the selective oxidative carboxylation of a conjugated diene with carbon monoxide in the presence of a catalyst to produce alkene dicarboxylic acid diester.